1. Field of the Invention
The invention relates to a method and means for monitoring a control apparatus having at least one actuating drive, a valve, a spindle, and a positioner.
The monitoring of control apparatuses for the ensuring correct operation thereof is of increasing importance, in the interest of lowering costs and increasing safety. Thus, for example, in order to increase efficiency it is desirable to prevent stoppage of process equipment due to component failure. For this purpose, timely exchange of components subject to wear is, for example, very important. However, premature exchange leads to unnecessarily high maintenance costs and temporary failures. For the optimization of maintenance and repair measures, it is thus necessary to carry out monitoring measures that provide information concerning the state of the equipment, in particular of the control apparatus and their components.
The possible sources of error can vary greatly, and comprise in particular the following known defects:
incorrectly tightened and worn-out sealings (e.g., stuffing box packings); PA1 broken spindles; PA1 broken springs (in the membrane drive); PA1 torn membranes (in the membrane drive); PA1 low supply pressure (in electropneumatic positioners); and/or PA1 contamination of the air filter (in electropneumatic positioners). PA1 a) determination of a first valve position at a first time; PA1 b) production of a force by the actuating drive, on the basis of changes in manipulated variables, for the well-defined, reproducible, uniform movement in a first direction, without stopping, of the spindle connected to the valve body of the valve; PA1 c1) modification of the manipulated variable for the actuating drive at the output of the positioner in such a way that a resulting force acts on the spindle that reverses the direction of movement of the spindle for the first time, whereby the changes in the manipulated variables are likewise well-defined, reproducible and uniform, but act in an opposed, second direction, and PA1 c2) determination of a second time, and of the second valve position appertaining thereto, when the first reversal of direction is triggered; PA1 d) acquisition of a third time at which, due to unavoidable delays, the spindle reaches a third valve position after the first reversal of direction, said third position standing in a first defined relationship to the second valve position; and PA1 e) definition and determination of a first time period between the second time and the third time; and PA1 f) comparison of the first time period with a corresponding first target value for the determination of the operativeness of the control apparatus. It is thereby inventively preferred to carry out the following steps as well: PA1 g) maintaining of the movement of the spindle after the first time period, and determination of a fourth valve position at a fourth time, as well as of a fifth valve position at a fifth time; PA1 h) calculation of a first speed of the movement from the fourth as well as the fifth time and from the fourth as well as from the fifth valve position; and PA1 i) comparison of the determined first speed with a corresponding second target value, in order to determine the operativeness of the control apparatus. In an embodiment of the invention, the additional execution of the following steps is provided: PA1 j) production of a force by the actuating drive on the basis of changes in the manipulated variables, for the well-defined, reproducible, uniform movement in the second direction, without stopping, of the spindle connected with the valve body of the valve, which second direction is opposed to the movement from the first time up to the triggering of the first reversal of direction; PA1 k1) modification of the manipulated variable for the actuating drive at the output of the positioner in such a way that a resulting force acts on the spindle, reversing the movement of the spindle for the second time, whereby the changes in the manipulated variables are likewise well-defined, reproducible and uniform, but act in the first direction, and PA1 k2) determination of a seventh time, and of the seventh valve position appertaining thereto, upon triggering of the second reversal of direction; PA1 k) acquisition of an eighth time, at which, due to unavoidable delays, the spindle reaches an eighth valve position after the second reversal of direction, said eighth position standing in a second determined relationship with the seventh valve position; PA1 m) definition and determination of a second time period between the seventh time and the eighth time; and PA1 n) comparison of the second time period with a corresponding third target value, in order to determine the operativeness of the control apparatus. It is thereby preferred that in addition the following steps are executed: PA1 o) maintaining of the movement of the spindle after the second time period, and determination of a valve position at a following time, as well as of a ninth valve position at a ninth time; PA1 p) calculation of a second speed of the movement from the stated following time as well as from the ninth time, and from the valve positions appertaining thereto; and PA1 q) comparison of the determined second speed with a corresponding fourth target value, in order to determine the operativeness of the control apparatus.
Some proposals for the monitoring of control apparatus are already known. EP 0 637 713 discloses a diagnosis system that uses a processor, connected with various sensors, and an intelligent positioner, in order to emit an alarm signal in case the measured values deviate from previously measured characteristics. The sensors detect structure-borne noise spectra and the pressure at a safety stuffing box, but however can also detect the flow at the valve, the input pressure and the valve position. A disadvantage of this method is that in order to acquire all the parameters, a large family of characteristics must be recorded and stored beforehand. This requires both a large expense for the determination of the parameters that characterize the correct function. As a result, a processor with a large memory is required.
In EP 0 708 389, a method is described in which the valve position and the manipulated variables are measured and stored. The data is then adapted to a defined mathematical model, and for each component at least one critical parameter is selected so that a faulty component of the control apparatus is found by means of adapting (fit) of the current measurement data to the mathematical model with the aid of the calculated critical parameter. However, it is disadvantageous that this method depends heavily on the selected mathematical model and the suitable selection of the critical parameters. The formal specification of the model and of the critical parameters additionally requires a suitable selection of parameters, in order both to make the adaptation soluble sufficiently rapidly and with a reasonable expense and also to select the critical parameters so that they are informative and sensitive to disturbances. In principle, the method resembles the selection of characteristic lines, with a tolerance band for the correct functioning of the control apparatus. The critical parameter thereby defines the tolerance, and the calculations become a mathematical variation problem in the matching. The solution of this matching, in particular given a larger number of critical parameters, becomes a demanding mathematical problem, depending on the type of model. Above all, process-oriented realization, as is often required directly at the control apparatus with intrinsically safe electronics, presents further problems, which render the method non-practicable in many cases.
DE 42 33 301 specifies a positioner that enables the recognition of faulty functioning of the positioner or control apparatus. For this purpose, the occurrence of a significant change in target value is detected, and the time required for the approximation of the actual value to a corresponding target value is monitored, so that the comparison of the actual value with the target value enables the recognition of faulty functioning. The possibility of likewise determining the type of error with a positioner of this type can however be realized only with difficulty. The reliability of the monitoring is also limited in that the approximation of the actual value to the target value requires relatively long time periods, and disturbances or changes in the target value present during these times falsify the result, or render it totally unusable.
Another method for monitoring a control apparatus with an electropneumatic positioner is specified in DE 44 19 548, in which a test signal whose amplitude can be modified is superposed on the control pressure. The state of the control apparatus is inferred from the relationship between this test signal and the movement of the system to be controlled. By superposition of the test signal, this method does indeed enable the monitoring of the hysteresis, which identifies above all friction at a cone rod. However, the initial position for the execution of this method has to be a constant valve position, in order to determine the beginning of a movement. The method is thus usable only beginning from a rest position, not from a valve movement.
The German patent application P 196 43 297.9, which does not enjoy prior publication, also discloses a method and a means for monitoring control apparatus, whereby the time difference between the driving of a drive switch unit and the possible response of a position detector, allocated to the previous rest position of the control apparatus, is monitored. The state of the control apparatus is inferred therefrom. However, similar to the method according to DE 44 19 548, the rest position of the control apparatus is an initial requirement, which disadvantageously limits the scope of application.